I have server on local network (POL 97), connecting with classic UO. I can easily authorize, but it hangs on "logging in to shard". When I take this PC and connect from internet (through NAT), everything works. Any ideas? Thanks!

I believe you need to define a "separate server" for LAN connections in servers.cfg. I had this entry in my servers.cfg for logging in on the same computer. You'll have to change the IP and IPMatch fields so they match up to your LAN. Name field doesn't matter and the port should be the same as the actual server.

Thanks, it works! I was also thinking to put server into separate vlan, this probably should help too, but it requires redesign of whole network NAT reflection didnt work.

Is there somewhere documentation, how this sharding works technically? I am in game alone, players are in different shard. What is happening technicaly? Does it run on different thread? Does this second shard have own instances of all NPCs? What happens during world save? Etc....Thanks.

PS: I used pc on the internet as proxy, but it also didnt helped me to get in. Why is POL behaving this way? Is there any reason for that?

There is no second shard unless you run a second instance of POL. Do you mean a different realm (map)? The second entry in the servers.cfg should still connect you to the same shard, it just goes through a different internet route. You can't connect from your public IP to your public IP directly which is what you were trying to do so the "second server" just takes a shortcut from one LAN IP to another LAN IP.